46 PEOCEEDINGS OF THE NATIONAL MUSEUM. vol.66. 



the crack on either side, which in the quarry shows as a narrow streak 

 with a black or greenish narrow median line, bordered by a lighter 

 colored zone. In the pegmatites, however, the alteration spread much 

 further from the crack supplying the solutions, and gave larger masses 

 of altered rock whose precise relation to fractures is rarely demon- 

 strable. In this section are also described some fracture fillings which 

 are not accompanied by alteration of the adjacent rock since the min- 

 erals of these fillings are the same as those which fill cracks elsewhere 

 accompanied by alteration of the walls. The agency producing these 

 phenomena is believed to have been solutions emanating as a final 

 magmatic product, following the differentiation of the several tex- 

 tural and compositional facies of the diabase. 



DIOPSIDE-FILLED CRACKS ACCOMPANIED BJ DIOPSIDIZATION OF THE ADJACENT DIABASE. 



• Many of the streaks in the diabase have a green central line which 

 consists of diopside filling the original open space of the crack, usually 

 accompanied by intergrown contemporaneous chalcopyrite and pyrite. 

 On either side of these diopside seams is a zone wherein the rock is 

 greenish in color. This border extends to a variable distance from 

 the central seam, usually of from 3 to 10 mm, making the total width 

 of the streak from 6 to 20 mm. In extreme cases these streaks may 

 be 30 mm. or more wide. One such streak of unusual width 

 accompanied the narrow aplitic dike previously described and simi- 

 lar alteration often accompanies other such dikes. This is regarded 

 as a coincidence rather than the result of the action of the aphte, 

 since the alteration may have taken place previous to the aplitic 

 injection along the crack later occupied by the aplite or subsequent to 

 the intrusion of the aplite along the cracks formed by reopening of 

 the same fissure. The greenish color of the altered rock is due to 

 replacement of the normal purplish augite by diopside, hence this 

 type of alteration is referred to as diopsidization of the rock. 



In thin section the diopside along the center of the streak is like 

 that of the adjacent wall rock, and there is usually a later very thin 

 crack filled with chlorite. There are crystals of titanite along the 

 crack. On either side of the fracture too and extending the entire 

 width of the altered streak, diopside has replaced the original augite 

 of the rock. Near the outer border of the altered area the replace- 

 ment may be actually observed, as remnants of the original brown- 

 ish augite remain in the centers of diopside crystals. Near the par- 

 ent augite the diopside is crowded with opaque inclusions, but for the 

 most part this pyroxene is clear and transparent and colorless to very 

 pale green in section. Cross sections of the prismatic crystals show 

 euhedral boundaries, well developed cleavage on (110) and twinning 

 on (100). They have not, however, inherited the basal parting of 

 the parent augite. 



